The present invention relates to a consumable welding element which can be used as filler metal for TIG welding or as the consumable electrode in MIG welding; however, it has more applicability to MIG welding of high strength metals of the type demanded in certain military and industrial applications and it will be discussed with respect to that application. This discussion is applicable to all uses of the invention.
In the art of electric arc welding it is common practice to use a MIG process wherein a metal wire is advanced toward the workpiece while an electric arc is created between the wire, referred to as a consumable electrode, and the workpiece for the purpose of melting the end of the wire or electrode to deposit metal onto the workpiece. Such MIG welding process involved highly developed and sophisticated technology in the area of the precise composition of the electrode which is consumed in the welding process. The composition of the electrode, the fluxing system, the type of electrical current, the shielding gas and other factors are adjusted and manipulated to obtain the desired welding objectives. One primary objective of any arc welding process is to obtain a high deposition rate which for the MIG process is usually indicated as a rate exceeding about 5 lbs/hr for welding in the vertical plane. The resulting weld metal must have a good bead formation, low amounts of slag formed over the weld bead, and other physical characteristics, which decrease the amount of labor needed to clean the resulting welded joint after the welding process. In addition, the weld metal, or bead, of the joint itself must have physical properties such as low cracking tendencies and high impact strength, sometimes measured as the Charpy impact value. In addition, it is desirable to be able to employ the welding electrode in not only a down hand position, but also welding out-of-position. All of these objectives have been the subject of substantial patented art. Details of electrode compositions need not be repeated for a person skilled in the art to understand the present invention.
As the yield strength of the workpiece metal increases to high strength levels, such as military specs HY-80, HY-100 and, even HY-130, serious problems have been experienced in obtaining low cracking and high impact strength while maintaining the ability to weld out of position at high deposition rates. When welding these high yield strength steels such as steels exceeding HY-80 (80,000 psi), the consumable electrode has heretofore normally taken the form of a solid metal wire with an out of position diameter generally about 0.045 inches and with the down hand diameter generally about 0.062 inches. Welding of high yield strength steels including and exceeding over HY-100 has been extremely difficult even with such solid MIG wire.
The object of the present invention is to provide a consumable electrode for MIG welding or filler wire for TIG welding, which can be used for high strength steels over HY-80 and ideally for steels with strength as high as HY-130. Efforts to accomplish this objective have, in the past, involved use of the solid wire with adjustments of the alloying composition together with preheating the workpiece. These efforts have been less than successful. In the welding art, there is always a tremendous impetus to improve the welding electrode for the purposes of increasing the ease at which high strength steel is welded with low cracking and high impact results. This is the objective accomplished by the present invention.
Attempts to use MIG welding for high yield strength steels (over 80,000 psi) with high deposition rates and high quality welding have, as explained, involved the use of solid wire. Such wire does have the capabilities of creating a relatively low diffusible hydrogen level in the weld metal; however, when a flux is required with the electrode, the diffusible hydrogen in the weld metal increases. This increases the tendency for the weld metal to crack upon solidification. In view of the tendency of fluxing compounds to increase the diffusible hydrogen in the weld metal, it bas been generally perceived that metal cored electrodes for the MIG process would not be satisfactory for high strength applications such as those previously mentioned. The processing and composition of the core materials also increased the amount of oxygen in the weld metal. Thus, when extremely high yield strength is employed, such as in the hull of ships, the military specifications have generally required the use of solid welding wire. This limited the out-of-position deposition rate to about 6 lbs/hr, which was obtainable only by elaborate electrical pulsing techniques. Irrespective of that, the military has not been satisfied with such solid welding wire for application above yield strengths of HY-100 steel. If flux cored electrode were employed, diffusible hydrogen of a higher level was experienced in the weld metal which increased the tendency of cracking. The use of solid welding wire for welding high yield strength steels became the normal and only acceptable practice. To increase the mechanical properties of the weld joint, the base metal was often heated preparatory to welding. Such solid wire electrode material also was compelled to contain the alloying materials in precise percentages. Consequently, accurate control over the alloying composition of the solid wire was extremely critical. This is a difficult metallurgical task.
In the welding industry, cored electrodes are quite commonly employed for MIG welding. Such cored electrode reduce the criticality of the alloying metal forming the tubular component of the electrode. This is accomplished by incorporating the alloying metals within the core in very precise proportions to obtain the desired alloying characteristics and percentages in the resulting weld metal. Although the use of a cored electrode with a metal powder core for controlling the composition of the weld metal does enhance the ability to control the actual composition of the weld metal, a metal cored electrode has a tendency to introduce a substantial amount of oxygen into the weld metal and cause a decrease in the impact strength of the weld metal. In addition, fluxing ingredients are required when using metal cored electrode in critical high strength applications. This results in higher diffusible hydrogen in the weld metal than was experienced in use of solid wire. In addition, such metal cored electrode material was somewhat sensitive to differences in electrical stick-out during the welding process. This disadvantage was attributed to moisture in the electrode core and seam and moisture in the shielding gas around the electrode. Thus, the use of metal cored electrodes with all of their advantages with respect to controlling the actual composition of the weld metal had serious deficiencies and were not employed for solving the problem of welding high yield strength steel of the type experienced in the ship building industry. In addition, cored electrodes were not generally employed in pulsed welding processes, such as employed in out of position welding of high strength steels.
Through the years the technology associated with welding high yield strength steel (over 80,000 psi) has involved a sophisticated MIG welding process using a solid welding wire. Suggestions to employ a metal cored electrode have met with a lack of acceptance. The use of metal powder alloys in the core of the electrode solved the criticality difficulties associated with precise percentages required in the weld metal; however, use of such a cored electrode increased the difficulties experienced with hydrogen and oxygen in the weld metal of the resulting joint. These disadvantages made use of metal cored electrode for high strength steel inappropriate. In view of the disadvantages involved in use of metal cored electrodes for MIG welding of high yield steel, it is not surprising that the use of flux cored electrodes has been limited for high yield strength welding. Problems recognized for metal cored electrodes were present in flux cored electrodes However, flux cored electrodes had still additional limitations.
A common flux cored electrode is one having a titanium dioxide fluxing system formed as powders in the core and surrounded by a low carbon steel sheath. Such flux cored electrodes provide good welding at high deposition rates, up to 8 lbs/hr, can be used out-of-position. However, such electrodes result in a higher level of diffusible hydrogen than can be employed in even HY-80 steel. In addition, the oxygen produces a relatively low Charpy strength. Consequently, the use of mild steel flux cored electrode employing the normal titanium dioxide fluxing system produces good welding characteristics, with high deposition rates and relatively high acceptance by the operator; however, the resulting weld metal has oxygen and hydrogen at a level which is not acceptable, for most high yield strength steels. Due to the extremely high quality and high deposition rates obtainable by other flux cored electrodes, it has been suggested to use a basic flux cored electrode employing such basic compounds as calcium fluoride. These electrodes do produce good impact strength, since the fluorine has a tendency to scavenge the oxygen from the weld metal; however, such basic flux cored electrodes can not be used satisfactory for out of position welding. Basic flux cored electrodes are not generally acceptable to the welder since their use is generally limited to down hand welding. Consequently, basic flux cored electrodes have not been employed for this type high yield strength welding.
In summary, metal cored electrodes, flux cored electrodes and basic flux cored electrodes have not heretofore been used acceptably in MIG welding of high yield strength steels, i.e. steels greater than about 80,000 psi (HY-80). Only solid wire electrode has been widely accepted in the military and industry for this high yield strength mechanized welding application.